This invention concerns an adapter device for electrodes which has an auxiliary reactance function on an electric arc furnace.
To be more exact the adapter device according to the invention is applied in cooperation with composite-type electrodes commonly used on electric arc furnaces, whether those furnaces are fed with alternating current or direct current.
In the description that follows, we shall speak of reactance, even though inaccurately, even when we are referring to furnaces fed with direct current.
The state of the art covers the various problems linked to electric arc furnaces, whether those furnaces are fed with alternating current or direct current, and in particular covers the problems relating to the secondary supply circuit of the furnace.
It is known that by secondary supply circuit of a furnace is meant the part of the circuit which goes from the transformer up to the electric arc.
Where the furnace is fed with alternating current, this part comprises:
a stationary connection between the secondary circuit of the transformer and the flexible cables, this connection being rigid and generally connected to the foundations; PA1 the flexible cables which connect the fixed connection with the transformer to the electrode-holder arms sand which enable the arms themselves to be moved vertically so as to alter the vertical position of the electrodes in relation to the furnace; PA1 the electrode-holder arms, of which there is one per each phase and which consist generally of a rigid cross-piece that can be displaced at least vertically by suitable actuation systems, the cross-piece bearing at its ends, on the one hand, the connection to the flexible cables and, on the other hand, the clamps for clamping the relative electrode; PA1 the electrodes, which are one per each phase and which generally consist of two or more consumable cylindrical graphite segments, from the ends of which the electric arc strikes.
When the furnace is fed with direct current, the secondary supply circuit of the furnace normally comprises, downstream of the secondary winding of the furnace transformer, a rectifier, reactors and a high-power connecting line that connects the reactors to the flexible cables associated with the single electrode-holder arm.
It is known in the art that a great component of the cost as compared to the total cost of the steel produced in a plant is represented by the quantity of graphite which constitutes the electrodes.
As the melting cycle progresses, the electrode is consumed, and it is necessary to lower the electrode progressively and to insert from above new segments in replacement to make up for that consumption.
These segments have to be processed beforehand on their outside by machine so that the clamps of the electrode-holder arms cooperate with a perfectly smooth surface in order to ensure the grip of the clamping and the correct vertical movement during the cycle.
This processing by machine entails not only a waste of material but also an appreciable increase of the unit costs of the graphite employed.
The consumption of graphite is measured in kgs/ton, that is to say, in kilograms of graphite per every ton of steel produced.
There is therefore the two-fold problem of reducing the quantity of graphite employed for embodiment of the electrodes and also of using graphite having a lower unit cost.
Another very important problem of which the manufacturers complain is the problem of ensuring an accurate balance in the three phases in furnaces fed with alternating current.
In other words, so as to obtain the best processing conditions, it is advisable to arrange that the three branches of the secondary circuit relating to the three phases have the same impedance and therefore deliver the same electric power.
This condition is necessary so as not to have imbalances in the currents circulating in the three phases with the resulting problems of overheating and creation of "hot points" in the furnace owing to an asymmetric distribution of the electric arc.
The greater the circulating currents are, the more important this problem becomes and often causes an overload in one of the external phases and a discharge in the other phases.
This occurrence which is known with the name "wild phase", leads to a lowering of the quality and efficiency of the melting and, above all, to a quick wear of the refractory material positioned on the side of the overloaded phase.
Moreover, it is especially difficult to achieve in the secondary circuit of the furnace a correct balance inasmuch as the geometric arrangement itself of the phases causes imbalances in the transmission of the power within the furnace.
Furthermore, in view of the very high value of the circulating currents, even relatively small imbalances between the different supply lines lead to great imbalances in the delivery of the power and therefore to the shortcomings mentioned above.
Besides, there exists the technological requirement of having conductors positioned as much as possible on the same plane in relation to the triangular connection which should ensure the balance of the reactances.
At the present time, to balance the phases, it is necessary to design accurately the individual components of the secondary circuit.
However, if mistakes take place in the calculation, fabrication and/or assembly of those components, such mistakes can no longer be put right without taking corrective action on the fixed part of the secondary circuit, and this entails high costs of the corrective action and long downtimes of the plant; moreover, such corrective action seldom ensures an efficient result.
In electric furnaces fed with direct current, in addition to the above problems linked to the consumption of graphite there exist further problems due to the great effect of the cost of the reactors on the overall cost of the steel produced.
PCT WO94/03028 discloses a proposal to position an additional reactor within the electrode-holder arm, thus entailing the result that the existing arms should all be replaced with heavy costs.
Furthermore, the placing of the additional reactor within the electrode-holder arms sets limits to the value of the reactor in view of the geometric limits of the arms.
Therefore, there is a substantial impossibility of placing additional reactors in pre-existing plants, for instance to increase the power delivered, without thereby entailing great modifications also to the civil works and vast and difficult operations of re-adaptation.
EP-A-0063711 describes a composite electrode comprising a cooled metallic support associated with the clamp of the electrode-bearing arm and a lower segment made of consumable graphite.
Between the metallic support and the graphite segment there is a double-layered element of heat insulation which peripherically encloses the cooling conduits which extend as far as the area where the metallic support is connected to the graphite segment.
The composite electrode as disclosed by EP'711 does not include any elements with a function of auxiliary reactance, nor does EP'711 include as a hypothesis the possibility of making this auxiliary reactance variable according to necessity.
GB-A-2 087 699 includes various solutions to achieve the connection between the metallic adapter and the graphite segment in a composite electrode for electric arc furnaces.
But this document too does not mention or include as a hypothesis the possibility of achieving an auxiliary reactance by using parts of the metallic adapter.
The adapter is achieved with a double-walled cylindrical structure, made of the same material, the only purpose of which is to define the annular cavity for the passage of the cooling fluid.